The ocean never stops moving. When you visit the beach, waves roll in and recede and the tides rise and fall. These are small daily changes that balance out over time. 

But over the past century, the average height of the sea has risen more consistently—less than a centimeter every year, but those small additions add up. Today, sea level is 5 to 8 inches (13-20 centimeters) higher on average than it was in 1900. That's a pretty big change: for the previous 2,000 years, sea level hadn't changed much at all. The rate of sea level rise has also increased over time. Between 1900 and 1990 studies show that sea level rose between 1.2 millimeters and 1.7 millimetersper year on average. By 2000, that rate had increased to about 3.2 millimeters per year and the rate in 2016 is estimated at 3.4 millimeters per year. Sea level is expected to rise even more quickly by the end of the century. 

Scientists agree that the changes in climate that we are seeing today are largely caused by human activity, and it's climate change that drives sea level rise. Sea level started rising in the late 1800s, soon after we started burning coal, gas and other fossil fuels for energy. When burned, these high-energy fuel sources send carbon dioxide up into the atmosphere. Carbon dioxide absorbs heat from the sun and traps it, warming the atmosphere and the planet. 

As the planet gets warmer, sea level rises for two reasons. First, warmer temperatures cause ice on land like glaciers and ice sheets to melt, and the meltwater flows into the ocean to increase sea level. Second, warm water expands and takes up more space than colder water, increasing the volume of water in the sea. 

Sea level rise will hit the coasts the hardest. Over the coming centuries, land that is today home to between 470 and 760 million coastal residents will be inundated by sea level rise associated with a 4 degree Celsius warming that will occur if we fail to curb the amount of carbon dioxide in the atmosphere. Much of this population lives in cities. Sea level rise already makes storms more dangerous, causing more flooding and damage in areas crowded with people. And it will affect different parts of the world differently, with some parts of the planet being particularly hard hit.

source : https://ocean.si.edu/through-time/ancient-seas/sea-level-rise